Semiconductor devices are built up using a number of material layers. Each layer is patterned to add or remove selected portions to form circuit features that will eventually make up an integrated circuit. Some layers can be grown from another layer; for example, an insulating layer of silicon dioxide can be grown over a layer of silicon by oxidizing the silicon surface. Other layers are formed using deposition techniques, typical ones being chemical vapor deposition (CVD), evaporation, and sputtering.
Deposition methods form layers using vaporized materials that condense to form a film on the surface of interest. Unfortunately, the films thus formed are not limited to the surface of interest, but tend also to form on other surfaces within the reaction chamber. Thus, after substantial use, a thick film of the deposited material accumulates on components and surfaces within the reaction chamber. These films eventually become troublesome sources of contaminants.
Etch processes also contaminate inside surfaces of reaction chambers. Etching involves applying a non-erodible photoresist mask to a surface such as a silicon or silicon dioxide layer, so that the areas of the layer not covered by the mask can be removed by exposure to an etching medium. However, the chemical reaction between the etchant and the underlying layer, or between the etchant and the surface of the reaction chamber and component parts, often produces byproducts that contaminate the surface of the chamber and internal components.
Semiconductor process equipment includes a reaction chamber with internal surfaces exposed to the process occurring within the chamber, and components within the chamber that are also exposed to the reaction process. In both deposition and etching processes, semiconductor process equipment with contaminated surfaces must be periodically cleaned or replaced.
Unfortunately, some forms of contamination are so stubbornly attached to the underlying material that removal of the contamination jeopardizes the part to be cleaned. There is a need for cleaning methods that minimize the damage to the chamber or component surface beneath the contamination.
For example, in a plasma etching process, the etchant is often a fluorine-containing gas. In the plasma, the gas breaks down, producing free fluorine radicals that attack the silicon. Examples of fluorine-containing gases used as etchants include halocarbon 14 (CF4), halocarbon 32 (CH2F2), halocarbon 116 (C2F6), chlorine trifluoride (ClF3), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3). The advantage of nitrogen trifluoride is that when it breaks down in the plasma, the only other element is nitrogen, an inert gas. With the halocarbons, there are free carbon molecules in the plasma; with chlorine trifluoride and sulfur hexafluoride, the other elements are chlorine and sulfur, respectively, which are both aggressive and potentially destructive elements. Thus, the industry is gravitating to nitrogen trifluoride.
Initially, chamber surfaces and components were typically made of aluminum, often with an anodized aluminum surface. However, the fluorine radicals produced in the etching process would attack the aluminum in the exposed surfaces of the chamber and internal components and these surfaces would accumulate a layer of aluminum fluoride contamination. This is because aluminum is easily oxidized and fluorine is an even more aggressive oxidizer than oxygen. The aluminum fluoride contamination limits the life of anodized aluminum parts. The contamination caused the surfaces to become rougher and the walls of the chamber and parts to become thinner as the conversion of aluminum to aluminum fluoride ate away at the underlying material. In addition, the reaction added another contaminant to the etch process—aluminum. A solution was needed to the problem of contamination of parts made of aluminum.
Companies that make the equipment used in the etching process, such as Tokyo Electron, Limited, or TEL, and Lam Research, Inc., graduated to using parts with a surface layer of sprayed-on ceramic for the surfaces exposed to the plasma etching process. Although ceramic is typically a form of aluminum oxide, or Al2O3, as is anodized aluminum, the sprayed-on ceramic surface was found to be much more durable than anodized aluminum, and seemed to be a good solution to the problem of contamination and deterioration of anodized aluminum. For a discussion of sprayed-on ceramics, see, e.g., U.S. Pat. No. 5,209,645, which is incorporated herein by reference.
However, the fluorine from the etchant in the plasma still had an affinity to bond with the aluminum in the ceramic. Chamber surfaces and parts coated with sprayed-on ceramic built up black aluminum fluoride contaminants that were extremely difficult to remove chemically. The presence of aluminum fluoride shortened the run life of the parts, and in addition the black aluminum fluoride would begin to particulate, adding a further contaminant to the etch process. A solution was needed to the problem of aluminum fluoride on aluminum-containing parts.
The semiconductor industry then began using process-exposed parts with a surface of sprayed-on yttrium oxide. Yttrium oxide lacks aluminum, and fluorine has little or no affinity for yttrium. Thus, sprayed-on yttrium oxide appeared to be a good solution to the problem of aluminum fluoride contamination. However, yttrium oxide is expensive. By way of rough comparison, sprayed-on ceramic parts cost approximately twice as much as parts with an anodized aluminum surface, and parts with a sprayed-on yttrium oxide surface cost approximately three times as much as anodized aluminum parts. Moreover, there are still lots of old parts with anodized aluminum or ceramic surfaces that are coated with aluminum fluoride. A need still exists for a reasonably cost-effective method of cleaning aluminum fluoride off of aluminum-containing substrates without significantly degrading the underlying material.